Cryogenic Distillation Method and Installation for Air Separation

ABSTRACT

The invention relates to a method and installation for separation of air by means of cryogenic distillation. According to the invention, all of the air is brought to a high pressure greater than the medium pressure and purified. Part of the purified air flow ( 11 ) is cooled in an exchange line ( 9 ) and, subsequently, divided into two fractions ( 13, 15 ). Each of the fractions expands in a turbine ( 17, 19 ), the intake pressure of the two turbines being greater than the medium pressure by at least 5 bars. Moreover, the discharge pressure of at least one of the two turbines is essentially equal to the medium pressure. At least part of the air that was expanded in at least one of the turbines is conveyed to the medium pressure column ( 100 ) of a double or triple column. Subsequently, a cold booster ( 23 ), which is mechanically connected to one ( 19 ) of the expansion turbines, draws the air which was cooled in the main exchange line and releases said air at a temperature greater than the intake temperature. The fluid thus compressed is reintroduced into the main exchange line, in which at least one part of the fluid condenses. In addition, at least one pressurised liquid ( 25 ) originating from one of the columns ( 200 ) is vaporised in the exchange line at an evaporating temperature and the turbine ( 17 ) which is not connected to the cold booster is connected to a booster ( 5 ) followed by a cooler.

The present invention relates to a process and to an installation forseparating air by cryogenic distillation.

It is known to produce a gas from pressurized air by vaporization ofpressurized liquid in an exchange line of an air separation unit by heatexchange with a compressed gas above a cryogenic temperature. Units ofthis type are known from FR-A-2 688 052, EP-A-0644388, EP-A-1014020 andpatent application FR 03/01722.

The energy efficiency of the known units is not excellent as the inflowof heat associated with the cryogenic compression has to be extracted.

In addition, in the case of schemes such as that of FIG. 7 of U.S. Pat.No. 5,475,980, the entire turbine coupled to the cold booster isassociated with an energy-absorbing system (oil brake) incorporated ontothe shaft of the machines and technologically limited to low powerlevels (around 70 kw).

Nevertheless, this type of process does appear to be economicallybeneficial, in particular when there is little energy reutilization orwhen energy is available at low cost. It is therefore potentiallybeneficial to be able to get round the technological limitation of theoil brake integrated onto the shaft of the turbine/booster assembly.

It is an object of the invention to propose an alternative that makes itpossible to achieve process schemes based on a cold booster but withoutan energy dissipation system integrated onto the turbine/booster shaft,and therefore to envisage using this scheme for practically all sizes ofair separation units.

One subject of the present invention is a process for separating air bycryogenic distillation in an installation comprising a double or tripleair separation column, the column of which operating at the higherpressure operates at what is called the medium pressure, and an exchangeline in which:

-   -   a) all the air is raised to a high pressure, optionally at least        5 bar above the medium pressure, and purified, optionally at        this high pressure;    -   b) one portion of the stream of purified air is cooled in the        exchange line and is then divided into two fractions;    -   c) each fraction is expanded in a turbine;    -   d) the intake pressure of the two turbines is (the intake        pressures of the two turbines are) at least 5 bar above the        medium pressure;    -   e) the delivery pressure of at least one of the two turbines is        substantially equal to the medium pressure;    -   f) at least one portion of the air expanded in at least one of        the turbines is sent to the medium-pressure column of a double        or triple column;    -   g) a cold booster mechanically coupled to one of the expansion        turbines takes in air, which has undergone cooling in the        exchange line, and delivers the air at a temperature above the        intake temperature, and the fluid thus compressed is        reintroduced into the exchange line in which at least one        portion of the fluid undergoes (pseudo)condensation;    -   h) at least one pressurized liquid coming from one of the        columns undergoes (pseudo)vaporization in the exchange line at a        vaporization temperature, and characterized in that:    -   i) the turbine not coupled to the cold booster is coupled to a        booster followed by a cooler; and, optionally,    -   ii) the intake temperature of the cold booster is close to the        (pseudo)vaporization temperature of the liquid.

According to other optional aspects of the invention:

-   -   the installation includes, in addition to the double or triple        column, a mixing column, and air coming from at least one of the        turbines is sent to the mixing column,    -   the air sent to at least one of the turbines upstream of the        mixing column comes from the booster other than the cold booster        and leaves this booster at a pressure above the high pressure;    -   air coming from at least one of the turbines is sent to the        bottom of the mixing column, in order to participate in mass        exchange; and    -   air at the high pressure is sent to a bottom reboiler of the        mixing column where it at least partially condenses before being        sent to the double or triple column.

Another subject of the invention is an installation for separating airby cryogenic distillation, comprising:

-   -   a) a double or triple air separation column, the column of        which, operating at the higher pressure, operates at what is        called the medium pressure;    -   b) an exchange line;    -   c) means for raising all the air to a high pressure, above the        medium pressure, and means for purifying it, optionally at this        high pressure;    -   d) means for sending one portion of the purified air stream into        the exchange line in order to cool it and means for dividing        this cooled air into two fractions;    -   e) two turbines and means for sending one air fraction to each        turbine;    -   f) means for sending at least one portion of the air expanded in        at least one of the turbines to the medium-pressure column of        the double or triple column;    -   g) a cold booster, means for sending air, preferably withdrawn        from an intermediate point on the main exchange line, to the        cold booster and means for sending air boosted in the cold        booster into the exchange line at an intermediate point upstream        of the withdrawal point;    -   h) means for pressurizing at least one liquid coming from one of        the columns, means for sending at least one pressurized liquid        into the exchange line, and means for expelling a vaporized        liquid from the exchange line; and    -   i) the cold booster is coupled to one of the turbines,

characterized in that the turbine not coupled to the cold booster iscoupled to an energy dissipation means comprising a booster followed bya cooler.

According to other optional aspects, the installation comprises:

-   -   a mixing column and means for sending air to the mixing column        from at least one of the turbines;    -   means for sending one portion of the air compressed in the        booster constituting the energy dissipation means, or forming        part of the latter, to at least one expansion turbine upstream        of the mixing column;    -   means for sending air, coming from at least one of the turbines,        into the mixing column in order to participate in mass exchange;        and    -   means for sending air at the high pressure into a bottom        reboiler of the mixing column and means for sending air at least        partially condensed in this bottom reboiler to the double or        triple column.

A complementary turbine will be used, operating in parallel with theturbine of the first turbine/booster assembly, and equipped with its ownenergy dissipation system. Favorably, this system will be a boosterfollowed by a water cooler installed in the warm part.

The expression “close in terms of pressure” means that the pressuresdiffer by at most 5 bar, preferably at most 2 bar. The expression “closein terms of temperature” means that the temperatures differ by at most15° C., preferably at most 10° C.

A booster is a single-stage compressor.

All the pressures mentioned are absolute pressures.

The term “condensation” includes pseudo-condensation. The term“vaporization” includes pseudo-vaporization.

This invention is distinguished from U.S. Pat. No. 5,475,980 in that, inFIG. 4 (optional turbine 9), the two turbines 8, 32 have very differentintake pressures, the difference being at least 14 bar, and in FIG. 5,the pressure difference is about 13 bar and a turbine exhausts at thelow pressure, this being prejudicial to the production of pure oxygen.

The invention will be described in greater detail with reference to thefigures in which:

FIGS. 1 and 2 show an air separation unit according to the invention.

In FIG. 1, a stream of air at atmospheric pressure is compressed toabout 15 bar in a main compressor (not illustrated). The air is thenoptionally cooled, before being purified (not illustrated) in order toremove the impurities. The purified air is divided into two portions.One portion 3 of the air is sent to a booster 5 where it is compressedto a pressure between 17 and 20 bar, and the boosted air is then cooledby a water cooler 7 before being sent to the warm end of the mainexchange line 9 of the air separation unit. The boosted air 11 is cooleddown to an intermediate temperature before leaving the exchange line andbeing divided into two fractions. Of course, it is possible that afraction of the stream 11 continues to be cooled until reaching the coldend of the exchange line 9, from where it will emerge liquefied. Afraction 13 is sent to a turbine 17 and the remainder—a fraction 15—issent to a turbine 19. The two turbines have the same intake temperatureand pressure and the same discharge temperature and pressure, but ofcourse it is possible for these temperatures and pressures to be closeto one another instead of being identical. The two streams output by theturbines are mixed together to form a stream 21 of air, a portion 121 ofwhich is sent to the double column and the remainder—a portion 122—issent to the mixing column 300. The stream 122 constitutes one portion ofthe stream 21 or optionally a fraction of the gaseous portion of thestream 21 in the case in which the latter is a two-phase stream. Ofcourse, it is possible to send the entire stream 21 to themedium-pressure column 100 and to withdraw therefrom a gaseous portion122 to be sent to the mixing column, the medium-pressure column in thiscase replacing the phase separator. The pressures of the medium-pressurecolumn and the mixing column may be different. As a variant, the turbine19 may be a blowing turbine delivering at the pressure of thelow-pressure column.

Another portion 2 of the air at 15 bar, constituting the remainder ofthe feed air, is cooled in the exchange line to an intermediatetemperature above the intake temperature of the turbines 17, 19,compressed in a second booster 23 up to about 30 bar and reintroducedinto the exchange line 9 at a higher temperature so as to continue itscooling.

Thus, the air 37 at about 30 bar liquefies in the exchange line andliquid oxygen 25 vaporizes in the exchange line, the vaporizationtemperature of the liquid being close to the intake temperature of thesecond booster 23. The liquefied air leaves the exchange line and issent to the column system.

A waste nitrogen stream 27 is warmed in the exchange line 9.

The first booster 5 is coupled to one of the turbines, 17 or 19, and thesecond booster 23 is coupled with the other of the turbines, 19 or 17.

The column system of an air separation unit is formed by amedium-pressure column 100 thermally coupled with a low-pressure column200 having a minaret, a mixing column 300, and an optional argon column(not illustrated). The low-pressure column does not necessarily have aminaret.

The medium-pressure column operates at a pressure of 5.5 bar, but it mayoperate at a higher pressure.

The air 121 coming from the two turbines 17, 19 is the stream sent intothe bottom of the medium-pressure column 100.

The liquefied air 37 is expanded in the valve 39 or, optionally, in aturbine, and sent to the column system.

Rich liquid 51, lower lean liquid 53 and upper lean liquid 55 are sentfrom the medium-pressure column 100 to the low-pressure column 200 afterin-valve expansion and subcooling steps.

Liquid oxygen is pressurized by the pump 500 and sent as pressurizedliquid 25 to the exchange line 9. Other liquids, whether pressurized ornot, may be vaporized in the exchange line.

Optionally, gaseous nitrogen is withdrawn from the medium-pressurecolumn and is cooled, again in the exchange line 9.

Nitrogen 33 is withdrawn from the top of the low-pressure column andwarmed in the exchange line, after having served for subcooling thereflux liquids.

Waste nitrogen 27 is withdrawn from a lower level of the low-pressurecolumn and warmed in the exchange line, after having served forsubcooling the reflux liquids.

Optionally, the column may produce argon, by treating a stream 51withdrawn into the low-pressure column 200. The stream 52 is the bottomsliquid sent from the argon column, if there is one.

The mixing column 300 is fed at the top with an oxygen-rich liquid 35,withdrawn from an intermediate level of the low-pressure column 200 andpressurized by the pump 600, and at the bottom with a stream 122 ofgaseous air coming from the turbines 17, 19. The mixing columnessentially operates at the medium pressure.

A gaseous oxygen stream 37 is withdrawn from the top of the mixingcolumn and then warmed in the exchange line 9, and a liquid stream 41 iswithdrawn as bottoms and sent to the low-pressure column after beingexpanded in a valve. It is possible to withdraw an intermediate streamfrom the column 300, which is sent to the low-pressure column.

In FIG. 2, a stream of air at atmospheric pressure is compressed toabout 15 bar in a main compressor (not illustrated). The air is thenoptionally cooled, before being purified (not illustrated) in order toremove the impurities. The purified air is divided into two portions.One portion 3 of the air is sent to a booster 5 where it is compressedto a pressure of between 17 and 20 bar, and then the boosted air iscooled by a water cooler 7 before being sent to the warm end of the mainexchange line 9 of the air separation unit. The boosted air 11 is cooleddown to an intermediate temperature before being divided into twofractions 103, 123. The fraction 103 leaves the exchange line and isagain divided into two fractions. One fraction 13 is sent to a turbine17 and the remainder—a fraction 15—is sent to a turbine 19. The twoturbines have the same intake temperature and pressure and the samedischarge temperature and pressure, but it is of course possible forthese temperatures and pressures to be close to one another instead ofbeing identical. The two streams output by the turbines are mixedtogether to form a stream 21 of air, one portion 121 of which is sent tothe double column and the remainder—a portion 122—is sent to the mixingcolumn 300. As a variant, the turbine 19 may be a blowing turbinedelivering at the pressure of the low-pressure column.

The fraction 123 continues to be cooled in the exchange line 9 and exitstherefrom upstream of the cold end to be sent to the bottom reboiler 301of the mixing column 300, where the fraction condenses, at leastpartially, in order to form the stream 125.

Another portion 2 of the air at 15 bar, constituting the remainder ofthe feed air, is cooled in the exchange line down to an intermediatetemperature above the intake temperature of the turbines 17, 19,compressed in a second booster 23 to about 30 bar and reintroduced intothe exchange line 9 at a higher temperature, so as to continue itscooling.

Thus, the air 37 at about 30 bar is liquefied in the exchange line andliquid oxygen 25 is vaporized in the exchange line, the vaporizationtemperature of the liquid being close to the intake temperature of thesecond booster 23. The liquefied air leaves the exchange line and issent to the column system after being mixed with the liquefied air 125coming from the reboiler 301.

A waste nitrogen stream 27 is warmed in the exchange line 9.

The first booster 5 is coupled with one of the turbines, 17 or 19, andthe second booster 23 is coupled with the other of the turbines, 19 or17.

The column system of an air separation unit is formed by amedium-pressure column 100 thermally coupled with a low-pressure column200 having a minaret, a mixing column 300, and an optional argon column(not illustrated). The low-pressure column does not necessarily have aminaret.

The medium-pressure column operates at a pressure of 5.5 bar, but it mayoperate at a higher pressure.

The gaseous air 21 coming from the two turbines 17, 19 is the streamsent to the bottom of the medium-pressure column 100.

The liquefied air 37 is expanded in the valve 39 and sent at least tothe medium-pressure column 100.

Rich liquid 51, lower lean liquid 53 and upper lean liquid 55 are sentfrom the medium-pressure column 100 to the low-pressure column 200 afterin-valve expansion and subcooling steps.

Liquid oxygen is pressurized by the pump 500 and sent as pressurizedliquid 25 to the exchange line 9. In addition or alternatively, otherliquids, whether pressurized or not, may be vaporized in the exchangeline.

Gaseous nitrogen is optionally withdrawn from the medium-pressure columnand is cooled, again in the exchange line 9.

Nitrogen 33 is withdrawn from the top of the low-pressure column and iswarmed in the exchange line, after having served to subcool the refluxliquids.

Waste nitrogen 27 is withdrawn from a lower level of the low-pressurecolumn and warmed in the exchange line, after having served to subcoolthe reflux liquids.

Optionally, the column may produce argon, by treating a stream 51withdrawn into the low-pressure column 200.

The mixing column 300 is fed only at the top with an oxygen-rich liquid35 withdrawn from an intermediate level of the low-pressure column 200and pressurized in the pump 600. The mixing column operates essentiallyat the medium pressure. By modifying the pressure of the stream 123, themixing column 300 may operate at a pressure different from the mediumpressure. Optionally, one portion of the rich liquid 51 may be sent tothe bottom of the column 300.

A gaseous oxygen stream 37 is withdrawn from the top of the mixingcolumn and warmed in the exchange line 9, and a liquid stream 41 iswithdrawn as bottoms and sent to the low-pressure column after beingexpanded in a valve.

1-8. (canceled)
 9. A process for separating air by cryogenicdistillation in an installation comprising a double or triple airseparation column (100, 200), the column of which operating at thehigher pressure (100) operates at what is called the medium pressure,and an exchange line (9) in which: a) all the air is raised to a highpressure, optionally at least 5 bar above the medium pressure, andpurified, optionally at this high pressure; b) one portion of the streamof purified air is cooled in the exchange line and is then divided intotwo fractions; c) each fraction is expanded in a turbine (17, 19); d)the intake pressure of the two turbines is (the intake pressures of thetwo turbines are) at least 5 bar above the medium pressure; e) thedelivery pressure of at least one of the two turbines is substantiallyequal to the medium pressure; f) at least one portion of the airexpanded in at least one of the turbines is sent to the medium-pressurecolumn of a double or triple column; g) a cold booster (23) mechanicallycoupled to one of the expansion turbines takes in air, which hasundergone cooling in the exchange line, and delivers the air at atemperature above the intake temperature, and the fluid thus compressedis reintroduced into the exchange line in which at least one portion ofthe fluid undergoes (pseudo)condensation; h) at least one pressurizedliquid coming from one of the columns undergoes (pseudo)vaporization inthe exchange line at a vaporization temperature, and i) the turbine (17)not coupled to the cold booster is coupled to a booster (5) followed bya cooler; and, optionally, j) the intake temperature of the cold booster(23) is close to the (pseudo)vaporization temperature of the liquid,wherein said installation includes, in addition to the double or triplecolumn, a mixing column (300), and air coming from at least one of theturbines (17, 19) is sent to the mixing column, optionally after havingpassed through the medium-pressure column (100).
 10. The process ofclaim 9, in which the air sent to at least one of the turbines (17, 19)upstream of the mixing column comes from the booster (5) other than thecold booster (23) and leaves this booster at a pressure above the highpressure.
 11. The process of claim 9, in which air (13, 15) expanded inat least one of the turbines (17, 19) is sent to the bottom of themixing column (300), in order to participate in mass exchange therein.12. The process of claim 9, in which air (123) at least at the highpressure is sent to a bottom reboiler (301) of the mixing column (300)where it at least partially condenses before being sent to the double ortriple column.
 13. An installation for separating air by cryogenicdistillation, comprising: a) a double or triple air separation column(100, 200), the column (100) of which, operating at the higher pressure,operates at what is called the medium pressure; b) an exchange line (9);c) means for raising all the air to a high pressure, above the mediumpressure, and means for purifying it, optionally at this high pressure;d) means for sending one portion of the purified air stream into theexchange line in order to cool it and means for dividing this cooled airinto two fractions; e) two turbines (17, 19) and means for sending oneair fraction to each turbine; f) means for sending at least one portionof the air expanded in at least one of the turbines to themedium-pressure column of the double or triple column; g) a cold booster(23), means for sending air, preferably withdrawn from an intermediatepoint on the main exchange line, to the cold booster and means forsending air boosted in the cold booster into the exchange line at anintermediate point upstream of the withdrawal point; h) means (500) forpressurizing at least one liquid coming from one of the columns, meansfor sending the at least one pressurized liquid into the exchange line,and means for expelling a vaporized liquid from the exchange line; i)the cold booster is coupled to one of the turbines (19); and j) theturbine (17) not coupled to the cold booster is coupled to a booster (5)followed by a cooler, wherein said installation includes a mixing columnand means for sending air to the mixing column from at least one of theturbines (17, 19).
 14. The installation of claim 13, which includesmeans for sending one portion of the air compressed in the booster (5)constituting the energy dissipation means, or forming part of thelatter, to at least one expansion turbine (17, 19) upstream of themixing column.
 15. The installation of claim 13, which includes meansfor sending air, coming from at least one of the turbines (17, 19), intothe mixing column in order to participate in mass exchange therein. 16.The installation of claim 13, which includes means for sending air (123)at least at the high pressure into a bottom reboiler (301) of the mixingcolumn (300) and means for sending air at least partially condensed inthis bottom reboiler to the double or triple column.